How long will the Sun last?

In the case of a star like our own Sun, we can figure that part out by looking at it and seeing how bright it appears in the sky, and then measuring how far away it is. And when you put those two things together, what you discover is that our Sun is sort of a run of the mill, medium-sized G-type star, and these stars tend to live for roughly 9-10 billion years. 

But to find out how much time the Sun has left, you need to know how old it is now. And you can’t just ask the Sun how old it is. So, what astronomers and geologists do is we turn to the rocks of the solar system and specifically the best kinds for age-dating the solar system are meteorites. And those give us an indication that the solar system is about 4.5 billion years old. So, when you take that into account with the fact that the Sun will live for about 10 billion years, we know we’ve got 4.5-5.5 billion years of fuel left in the Sun. 

But what happens when the Sun starts to run out of fuel? Up until this point, the Sun has been relatively stable with a very nice balance between two forces. There’s the inward force of gravity, which is trying to force the Sun together, and the outward force of radiation pressure, which is pushing outward due to reactions that are going on at the core of the Sun. And as the Sun dies, as it’s running out of fuel, you start to lose that balance. And initially, radiation pressure decreases, and the Sun starts to collapse in on itself, decreasing and shrinking in size. 

And when that happens, the core of the Sun gets hotter. You get another phase of burning and the Sun starts to expand again, and the Sun will move into this phase that we call the red giant phase which will encompass Mercury and Venus and spread all the way out to where the Earth is right now. Ultimately there’s just not enough fuel to sustain it, and so gravity collapses the sun down to a fraction of its size, of its temperature, of its brightness, and turns it into this object that we call a white dwarf, that’s really dense and has a strong gravitational field, but it’s faint, and would not be visible to the human eye. Everyone wants to know how much longer the Sun has to go because without a Sun it really means big trouble for the Earth and for life on Earth. But you don’t have to worry because we’ve got plenty of time. Hopefully we’ll be out exploring the cosmos and looking for new worlds by the time the Sun runs out of fuel.